The invention relates to the design and a process for the manufacture of multiple piece rolls, in particular backup rolls, for the use in metal rolling e.g. rolling of steel or aluminium. Typically, in known systems, backup rolls are used to support the work rolls in a rolling mill. It is desirable to keep the work roll diameter small because this keeps the rolling loads and torques as low as possible. However, a small diameter work roll bends easily under the rolling load. The larger diameter backup roll is used to support the work roll and minimise the bending of the rolls under load. A 4-high mill stand comprises 2 work rolls and 2 backup rolls but the same principle also applies to other mill configurations such as 6-high mills.
In most modern rolling mills, the backup roll is manufactured as a single piece either by forging or by casting. For large rolling mills such as plate mills, the backup roll can be more than 2 meters in diameter, over 10 meters long and weigh 150 tonnes or more.
In the case of a cast roll, a double-pour casting technique is normally used in which the barrel is made from a different material to the necks and core of the roll. Spin casting techniques are often used to try to obtain homogenous properties. With very large rolls it is quite difficult to avoid porosity in the casting along the centreline of the roll due to shrinkage during cooling. Due to the large size and the difficulty in avoiding defects and flaws there are very few companies who can manufacture cast backup rolls for large plate mills.
In the case of a forged roll, the normal forging process for a roll starts with an ingot diameter which is typically 2 or more times greater than the final diameter of the roll. A large reduction in the diameter during forging is required to ensure that the material does not have any flaws or defects particularly in the centre of the roll. In order to forge the roll with a large forging reduction ratio an ingot which is much larger than the final roll size is required. In addition, the ingot is usually pressed axially in a process called upsetting in order to increase its diameter further. In order to achieve a large forging reduction the ingot size and weight are very large and consequently there are very few companies who can manufacture these large rolls.
Due to the difficulty in manufacturing large backup rolls, there is considerable interest in being able to manufacture these rolls as multiple smaller pieces instead of as a single piece.
In the past, some backup rolls have been manufactured as two pieces consisting of a sleeve or shell and an inner core. Various methods have been used for attaching the shell or sleeve to the core of the roll including shrink fitting, tapers, reversed-tapers and keys. However, these designs have not been generally accepted by the industry. One of the fundamental problems is that it is very difficult to ensure that the sleeve and the core do not slide relative to one another when the roll is loaded. When the roll is loaded it bends and the sleeve or shell tends to slide relative to the core or to separate from the core. Since the direction of the deflection of the roll changes every revolution this leads to rapid wear and damage. In addition, any sliding or separation between the sleeve and the core results in the roll exhibiting a non-linear stiffness. This causes major problems for the thickness control systems used on most rolling mills.